1. Field of the Invention
This invention relates to a process for the production of a diaphragm for electrolytic cells used for the electrolysis of, for example, alkali metal halides in an aqueous solution.
An object of this invention is to provide a process that renders the formation of a stratified diaphragm more uniform, wherein the diaphragm is precipitated or deposited from a suspension having a high solids content of diaphragm material onto a hollow diaphragm support.
2. Review of Prior Art
Such diaphragms are manufactured according to the following exemplary method, which will be explained with reference to a diaphragm suitable for alkali chloride electrolysis; the diaphragm is in this case applied to a diaphragm support which is subsequently connected as the cathode in an electrolytic cell (cathode--supported diaphragm).
The diaphragm support is a hollow body exhibiting several plate-shaped grid surfaces (e.g. course wire grids) for the deposition of the diaphragm material. The area of the grid openings ranges from 0,04 cm.sup.2 to 4 cm.sup.2. The inner spaces or pockets of these plates are in communication with one another and form the hollow chamber of the cathode containing the catholyte in the electrolyte bath. The anolyte is located on the outside of the greatly segmented diaphragm surface in the electrolyte bath. The hollow body generally has two openings, a smaller one for the catholyte outlet and a larger one from which the hydrogen escapes if the finished, coated diaphragm support is used as a cathode in the electrolyte bath.
A blend of short and long asbestos fibers, for example chrysotile asbestos, which blend can optionally contain fibers of organic, synthetic polymers or other organic and inorganic additives, is made into a slurry in a sodium-chloride-containing, dilute sodium hydroxide solution corresponding to the composition of the cell liquor and containing a small quantity of surfactants, and is introduced into a basin of adequate size. The solids content of the slurry ranges from 0,1% to 2,5%.
A suction pump is connected via a suction hose to the catholyte outlet of the still uncovered diaphragm support, and the diaphragm support is suspended in the basin with the slurry of the diaphragm material, the diaphragm support being completely submerged in the slurry. The plate-shaped grid surfaces are disposed vertically. With the aid of the suction pump, the slurry is sucked through the grid surfaces of the diaphragm support and pumped into a collecting vessel. The diaphragm material is deposited on the grid surfaces and there builds up the stratified (layer-type) diaphragm, which diaphragm is densified by the liquid flow on the grid surface. The pumped-off slurry is used later on for preparation of the next bath or suspension.
The completely covered or coated diaphragm support is dried in an oven and--depending on the type of materials added to the slurry--treated at elevated temperature, whereby the diaphragm is inherently stabilized. This completes the coating process for the diaphragm support, and a ready-for-use diaphragm is provided. Details on the process are contained, for example, in DAS Nos. 2,401,942, 2,608,398, 2,756,720, 2,834,556; U.S. Pat. No. 4,180,449; European Pat. No. 1,664, and European Pat. No. 18,034.
The type and progression of accretion of diaphragm material on the diaphragm support are of decisive importance for the properties of the diaphragm during the subsequent electrolytic process in the electrolytic cell. As long as the diaphragm has an area of up to about 0.2 m.sup.2, as utilized for electrolytic cells on a laboratory scale, it is easy to apply a uniform diaphragm layer. In contrast, industrial-scale cells exhibit a diaphragm area of up to 60 m.sup.2. In such large diaphragms, differences are repeatedly encountered in structure and characteristic of the diaphragm, within one of these large-sized coatings as well as between several diaphragms manufactured in succession under unchanged conditions. Such differences cause adverse effects in the degree of current efficiency of the electrolytic cells, in their operating life, and in the composition of the electrolysis products.
The desired, unifrom diaphragms are characterized by a maximally constant layer thickness and a maximally constant packing density of the diaphragm material, as well as by a maximally constant flow resistance over the entire surface area.
These conditions are to be complied with within the diaphragm on a diaphragm support as well as between the diaphragms on several supports; this can be achieved only by means of a sufficiently uniform and reproducible accretion procedure and heretofore has not been satisfactorily realized in production of large-sized diaphragms.
Consequently, an object of this invention is to effectively improve the accretion process of the diaphragm material on the diaphragm support.